EFI engines in automobiles are commonly controlled by an on-board computer, typically a microprocessor-based device, which controls the timing and duration of the fuel injection in response to operational parameters sensed by a number of sensors on the engine, e.g. temperature, engine speed, throttle position, air flow, etc. These operational parameters can be measured many times per second so that the engine is continually operated at optimum efficiency.
Many microprocessor or micro-computer based control circuits for EFI engines are programmed to accept measured values of sensed operational parameters only when such values fall within a predetermined range e.g. to avoid responding to spurious signals or to avoid acting on faulty measurements. If the value of an operational parameter as measured by a particular sensor is outside the predetermined range, the sensor may be judged by the computer control system to have failed (whether this is, in fact, correct or not), and the actual output signal of the sensor may be replaced by a standard value (as described, for example, in U.S. Pat. No. 4,780,826) . The use of a default value enables the engine to keep operating despite the failure of a sensor. However, although the engine will still operate, it will not perform as efficiently as it should. Since the fault in the engine may be masked by the default value inserted by the computerised electronic control system, it IS difficult, if not impossible, for mechanics to locate and correct the fault using conventional tools.
Complex and expensive diagnostic equipment is PG,3 normally required to locate the fault. Such equipment is often computer-based, requiring a computer device manufactured specifically for that particular application. The use of such complex and specialized diagnostic equipment and the need for trained technicians result in increased costs for motor vehicle repair.
It is the object of the present invention to provide apparatus for computer-aided diagnosis of EFI engines which is within the economic and technical reach of most motor mechanics.
It is a further object of the invention to provide a method of computer-aided diagnosis of EFI engines wherein a diagnostic computer program includes tutorial information to enable such diagnosis to be performed by most motor mechanics.